I’ve got an 89 420SEL with 115,000 miles on it. It’s been in the family since new and it’s mine now.

I’ve noticed what sounds like “gear noise” when driving at 20-25 mph. It’s just a steady “Rrrrrrrrrrrrrrrrrrrrrrrrrr” type sound. Not too loud but noticeable to paranoid, anal retentive freaks like me. It does increase in pitch as the cars speed increases and on the highway at 70mph it’s a very high-pitched whine that fades in and out….(I’m just assuming that the two sounds are related.)

The noise is not affected by engine load, or which gear the car is in. While coasting if I shift to neutral, the noise remains unchanged.

This leads me to believe that the noise is coming from the differential, not the transmission. Would I be correct in this assumption?

I had planed on putting in some 80w140 synthetic gear oil but I just wanted to get some opinions on the subject before I do anything.